1. Field of the Invention
The present invention relates to an auto feedback and photo attenuation structure of a vertical cavity surface emitting laser, consisting of a vertical cavity surface emitting laser (VCSEL) and a monitor photodetector (MPD).
2. Description of the Related Art
A conventional vertical cavity surface emitting laser (VCSEL) 10 in accordance with the prior art shown in FIG. 1 comprises an active region 11 having a first side provided with a P-distributed bragg reflector (P-DBR) 12, and a second side provided with a N-distributed bragg reflector (N-DBR) 13, and a substrate 14 mounted on the bottom of the N-DBR 13. After output of the light, it resonates between the P-DBR 12 and the N-DBR 13 to emit laser light.
Recently, the VCSEL 10 having a short wavelength (850 nm) has been used broadly. The substrate 14 of the VCSEL 10 having a wavelength of 850 nm is often made of GaAs (Gallium Arsenic) which will absorb the light entering the substrate 14, so that the VCSEL 10 may emit light on the top face only. Thus, when the feedback work of an auto power control (APC) is performed, the process of the feedback work of the VCSEL 10 is more difficult than that of the conventional edge emitting laser which emits light at two faces.
The method of the APC of the conventional VCSEL often uses a monitor photodetector (MPD) that is attached on the top, the bottom and the side of the conventional VCSEL.
The conventional VCSEL on the top of which the MPD is mounted is disclosed in the following patents: U.S. Pat. No. 5,136,603; U.S. Pat. No. 5,606,572; U.S. Pat. No. 5,577,064; U.S. Pat. No. 5,742,630; and U.S. Pat. No. 5,751,751.
The conventional VCSEL on the bottom of which the MPD is mounted is disclosed in the following patents: U.S. Pat. No. 5,491,712; U.S. Pat. No. 5,606,572; U.S. Pat. No. 5,663,944; U.S. Pat. No. 5,732,101; U.S. Pat. No. 5,577,064; and U.S. Pat. No. 5,801,402.
The conventional VCSEL on the side of which the MPD is mounted is disclosed in the following patents: U.S. Pat. No. 5,285,466; U.S. Pat. No. 5,491,712; U.S. Pat. No. 5,606,572; U.S. Pat. No. 5,648,979; U.S. Pat. No. 5,663,944; U.S. Pat. No. 5,577,064; U.S. Pat. No. 5,757,836; and U.S. Pat. No. 5,974,071.
The conventional VCSEL on the reflector of which the MPD is mounted is disclosed in the following patents; U.S. Pat. No. 5,475,701; U.S. Pat. No. 5,606,572; U.S. Pat. No. 5,577,064.
In the above-mentioned patents, the reflectivity of the reflector of the conventional VCSEL may be adjusted to provide proper output light to the MPD to function as the basis of the APC. However, the process of the epitaxy growth layer of the conventional VCSEL cannot be controlled easily. If the MPD structure grows therein, the manufacturing process is more difficult, and relatively, the output power obtained by the MPD cannot be controlled easily, so that the work of the APC is more difficult.
With regard to application of the VCSEL having a longer wavelength (1300 nm), it is limited to the problem of the material system, such that the VCSEL having the wavelength of 1300 nm has not been used broadly.
The light having a wavelength of 850 nm and 1300 nm is an infrared light that cannot be seen by the people. Thus, for consideration of eye safety, the output of the laser light must satisfy the requirement of the eye safety class I. That is, the output of the laser light must satisfy the requirement of 200 to 400 xcexcW. Thus, the VCSEL whose output power usually exceeds 1 mW has to be attenuated properly so as to satisfy the requirement of the eye safety class I.
The present invention is to provide an auto feedback and photo attenuation structure of a vertical cavity surface emitting laser, wherein the thickness of the absorbing layer of the MPD may be adjusted so as to adjust, in the light output power of the VCSEL having a wavelength of 850 nm and 1300 nm, the power absorbed by the absorbing layer and the power penetrating the MPD, thereby functioning as a feedback path of an auto power control, and satisfying the requirement of the output power of the eye safety class I.
The primary objective of the present invention is to provide an auto feedback and photo attenuation structure of a vertical cavity surface emitting laser (VCSEL) including a VCSEL combined with a monitor photodetector (MPD). The thickness of the absorbing layer of the MPD may be adjusted, so that when output light of different wavelengths passes through the MPD, a portion of light may be absorbed by the absorbing layer to function as a feedback path of an auto power control,; and a portion of the light may penetrate through the absorbing layer, and may satisfy the requirement of the eye safety class I. The thickness of the absorbing layer of the MPD may be controlled precisely, thereby increasing the quality of products and decreasing cost of fabrication.
The MPD includes a substrate and an absorbing layer. For the VCSEL having a wavelength of 1300 nm, the light may pass conveniently, so that the substrate may be transparent. Thus, the purpose of adjusting the power absorbed by the absorbing layer and the power penetrating the MPD may be achieved by adjusting the thickness of the absorbing layer.
For the VCSEL having a wavelength of 850 nm, the light cannot penetrate through the substrate, so that the substrate needs to be etched with an etching cavity which may co-operate with adjustment of the thickness of the absorbing layer, so that a portion of light may be absorbed by the absorbing layer to function as a feedback path of an auto power control, and a portion of the light may penetrate through the etching cavity.